It has long been common practice in the manufacture of mass produced parts such, for example, as carburetor housings, automatic transmission housings, cylinder blocks and the like to mount the part on a pallet, fixture or a jig and while so mounted move the part through a number of work stations. At each work station one or more machining operations are performed and the part is then transported to another work station where additional machining operations are performed. The parts mounted on their pallets, fixtures or jigs are transported from work station to work station on conveyors and remain on the conveyor or a spur track of the conveyor at each work station. This arrangement has a number of drawbacks and limitations.
One of the significant drawbacks is that such systems are static in that they normally have to be substantially or entirely rebuilt to adapt them from a part of one design to a part of another design. In many cases there is little or nothing that can be salvaged from the entire system. Thus, these systems are excessively costly and are limited to use only for parts which are to be mass produced over a substantial period of time.
Another important drawback is that many of these fixtures provide very limited tool access. If the fixture permits machining from more than one face, frequently fixture design does not permit accurate indexing of the part to the tools. In many cases, when the part must be approached for machining from several faces, the part has to be removed from its fixture and remounted on a different fixture. Not only is this procedure time consuming and labor intensive, it also requires the designing, building, testing and maintenance of several fixtures to complete the machining of a single part. Further, of equal if not greater significance, is the fact that the use of multiple fixtures or the like can and does result in an accumulation of tolerances sufficient to render a significant proportion of the resulting parts marginal or scrap. Further, even though the fixtures were initially sufficiently accurate that acceptable parts were originally produced, differentials in rate or wear and other factors can rapidly eliminate this situation.
Another factor adversely affecting the accuracy of many conventional machining systems is the absence of a single, uniform reference or index point capable of accurately positioning the part of all work stations and at all positions in each work station. This is often true in pallet type systems because the pallets lack a uniform reference point common to all the pallets. Another major source of inaccuracies in the finished parts is that resulting from clips and turnings which become lodged between the fixture and its supporting surface, misaligning the part with respect to the tool.
Another factor which contributes to lack of accuracy in conventional pallet type machining systems is that each pallet, fixture or jig normally is custom designed for the part with which it is to be used. This is often true of the several fixtures frequently necessary to machine a single part in free transfer systems. Thus, there is no reference point common to all the fixtures for indexing them to the machine tools which are to work on the part. This has materially handicapped efforts to maintain the accuracy necessary for true interchangeability.